The present application is based on Japanese priority applications No. 11-220649 filed on Aug. 4, 1999, No. 11-229794 filed on Aug. 16, 1999, No. 11-243745 filed on Aug. 30, 1999, No. 11-339267 filed on Nov. 30, 1999, No. 2000-057254 filed on Mar. 2, 2000, and No. 2000-144604 filed on May 12, 2000, the entire contents of which are hereby incorporated by reference.
The present invention generally relates to semiconductor devices and more particularly to semiconductor light-emitting devices and laser diodes.
Particularly, the present invention relates to a laser diode operable in a wavelength range of 630-680 nm. Further, the present invention relates to a laser diode for use in optical recording and optical reading of information or light-emitting display of information. Further, the present invention relates to a semiconductor light-emitting device based on a III-V compound semiconductor material.
Further, the present invention relates to a vertical-cavity laser diode suitable for an optical source of optical recording and reading of information or light-emitting display of information. The present invention further relates to an optical information recording apparatus such as a xerographic image recording system or an optical system and optical telecommunication system including an optical interconnection device that uses a vertical-cavity laser diode.
In these days, efforts are being made to develop a red-wavelength laser diode operable in the wavelength range of 630-680 nm as an optical source of optical disk recording apparatuses. Such an optical disk recording apparatus includes a DVD (Digital Video Disk or Digital Versatile Disk) player. The laser diode is used in such disk recording apparatuses as the optical source for reading and/or writing of information.
In order to increase the writing speed of information into the optical disk in such optical disk devices, it is necessary to increase the output power of the laser diode used therein.
Hereinafter, a brief review will be made on conventional red-wavelength laser diodes.
FIG. 1 shows the cross-sectional diagram of a conventional red-wavelength laser diode of an AlGaInP system disclosed in the Japanese Laid-Open Patent Publication 11-26880.
Referring to FIG. 1, a substrate 1 of n-type GaAs carries thereon a buffer layer 2 of n-type GaAs, a cladding layer 3 n-type AlGaInP, a quantum well active layer 4 including therein alternate and repetitive stacking of an AlGaInP layer and a GaInP layer, a cladding layer 5 of AlGaInP of low carrier concentration (2xe2x88x926xc3x971017 cmxe2x88x923), and an etching stopper layer 6 of p-type GaInP.
Further, there is provided a ridge structure 10 on a part of the etching stopper layer 6 wherein the ridge structure 10 includes a carrier-diffusion suppressing layer 7 of p-type AlGaInP, a cladding layer 8 of p-type AlGaInP, and a band-discontinuity relaxation layer 9 of p-type GaInP. Further, there are formed a pair of electric current blocking regions 11 of n-type GaAs on the surface part of the etching stopper layer 6 where the ridge structure 10 is not formed, and a contact layer 12 of p-type GaAs is formed continuously on the current blocking regions 11 and the band-discontinuity relaxation layer 9 therebetween. The contact layer 12 carries thereon a p-type electrode 13, and an n-type electrode 14 is formed on the bottom surface of the substrate 1.
In the laser diode of FIG. 1, there occurs a current confinement in the ridge structure 10 wherein the ridge structure 10 provides a current path between the current-blocking regions 11, and the electric current is confined into the ridge structure 10 thus formed of p-type GaAs. Further, it should be noted that the current-blocking regions 11 absorb the optical radiation from the quantum well active layer 4 and there is induced a refractive-index difference between the ridge structure 10 and the region outside the ridge structure 10 as a result of such an optical absorption. Thereby, there occurs an optical confinement in the ridge structure 10.
Such a ridge structure 10, while being able to form so-called optical loss-guide structure in the laser diode, has a drawback in that it increases the threshold current of laser oscillation due to the optical absorption caused by the current-blocking regions 10.
FIG. 2 shows the cross-sectional structure of a red-wavelength laser diode disclosed in the Japanese Laid-Open Patent Publication 9-172222.
Referring to FIG. 2, the laser diode is constructed on a substrate 15 of n-type GaAs and includes a buffer layer 16 of n-type GaAs, a cladding layer 17 of n-type AlGaInP, an active layer 18 of GaInP, a cladding layer 19 of p-type AlGaInP and an intermediate layer 20 of p-type GaInP, wherein the layers 16-20 are formed on the substrate 15 consecutively by an epitaxial process.
In the intermediate layer 20, there are formed a pair of stripe grooves reaching the p-type cladding layer 19, and the stripe grooves thus formed define a stripe ridge 21 therebetween. Further, current-blocking regions 22 are formed by filling the stripe grooves with a layer of n-type AlGaAs, and the entire structure is covered by a cap layer 23 of p-type GaAs formed by an epitaxial process.
In the case of the laser diode of FIG. 2, the current-blocking regions 22 are formed of AlGaAs having a bandgap larger than a bandgap of the active layer 18. For example, the current-blocking regions 22 are formed to contain Al with a concentration of 39% in terms of atomic percent when the laser diode is designed to operate at the wavelength of 650 nm. In the case the laser diode is to be operated at the wavelength of 630 nm, the Al content in the current-blocking regions 22 should be 45% or more in terms of atomic percent. In such a case, the current-blocking regions 22 are transparent to the laser beam and the loss at the optical waveguide is minimized.
FIG. 3 shows the cross-sectional diagram of a red-wavelength laser diode disclosed in the Japanese Laid-Open Patent Publication 7-249838.
Referring to FIG. 3, the laser diode is constructed on a substrate 24 of GaAs and includes, on the substrate 24, a cladding layer 25 of n-type AlGaInP having a composition (Al0.6Ga0.4)0.5In0.5P, an active layer 26 having a quantum well structure formed by an AlGaInP barrier layer and a GaInP quantum well layer, an inner cladding layer 27 of p-type AlGaInP having a composition of (Al0.6Ga0.4)0.5In0.5P, an etching stopper layer 28 of p-type GaInP having a composition of Ga0.5In0.5P, an outer cladding layer 29 of p type AlGaInP having a composition (Al0.6Ga0.4)0.5In0.5P, a buffer layer 30 of p-type GaInP having a composition of Ga0.5In0.5P, and a cap layer 31 of p-type GaAs.
The laser diode is formed with a mesa structure by a wet etching process, wherein the wet etching process is conducted while using an SiN mask formed on the cap layer 31 with a width of 6 xcexcm, until the etching stopper layer 28 is exposed. After the mesa structure is thus formed, a pair of current-blocking regions 32 of n-type AlInP and a pair of cap regions 33 of n-type GaAs are formed on the mesa surface. Thereby, the current-blocking regions 32 are grown so as to have a composition of Al0.5In0.5P on the part making contact with the mesa surface. After removing the SiN mask, a contact layer 34 of p-type GaAs is formed so as to cover the cap regions 33, the current-blocking regions 32 and the cap layer 31 on the mesa structure.
In the laser diode of FIG. 3, too, the problem of waveguide loss is avoided due to the large bandgap energy of AlInP used for the current-blocking regions 10. Further, the use of the AlInP current-blocking regions 32 is advantageous in view of the fact that AlInP has a smaller refractive-index as compared with the inner and outer cladding layers of p-type AlGaInP. Thereby, it should be noted that there is formed a real refractive-index difference between the region inside the ridge and the region outside the ridge, and a real refractive-index waveguide is formed in the laser diode.
In the laser diode of FIGS. 2 and 3, it should be noted that the current-blocking regions 22 or 32 contain an increased amount of Al for minimizing the optical absorption by the current-blocking regions. As noted already with reference to FIG. 2, the Al content in the current-blocking region 22 of AlGaAs has to be set to 39% or more in atomic percent when the laser diode is to be operated at the wavelength of 650 nm. In the case of the laser diode of FIG. 3, on the other hand, the current-blocking region 32 contains Al with an amount of 50% in terms of atomic percent in the vicinity of the mesa surface, while this value of Al concentration is larger than the Al concentration (35% in atomic percent) of the AlGaInP cladding layer typically used in an AlGaInP laser diode. When the Al content in a semiconductor layer is large as such, there tends to occur a problem of optical damaging at the edge surface of the laser optical cavity due to non-optical recombination of carriers. It should be noted that the increase of Al content tends to increase surface states, while the surface states tend to facilitate the non-optical recombination of carriers.
Thus it is an object of the present invention to provide a red-wavelength laser diode having a reduced optical waveguide loss and simultaneously a reduced optical damage at the edge surface of the optical cavity formed in the laser diode.
As noted already, the laser diode of the AlGaInP system is becoming an important target of investigation in relation to application to laser beam printers, optical disk drives and the like, due to the fact that the laser diode of this system can produce an optical beam with the wavelength range of about 600 xcexcm.
In the application to the optical source of disk drives, it is required that the fundamental mode of laser oscillation is a horizontal lateral mode of single peak. Further, it is required that astigmatism is small.
Such a single fundamental mode laser oscillation with reduced astigmatism is realized by using a real refractive-index waveguide structure, and there is proposed a visible-wavelength laser diode structure based on an AlGaInP system as represented in FIG. 4.
Referring to FIG. 4, the laser diode is constructed on a substrate 42 of n-type GaAs and includes a cladding layer 43 of AlGaInP, an active layer 44 of GaInP, and a cladding layer 45 of AlGaInP, formed consecutively on the substrate 42.
After forming the cladding layer 45, a ridge stripe of an inverse-mesa structure is formed so as to extend axially, and high-resistance regions 46 of AlInP are formed at both lateral sides of the stripe structure by causing a selective growth process while using an SiO2 mask on the stripe region. Further, a GaInP layer 48 and a p-type GaAs layer 49 are grown selectively and consecutively on the AlGaInP layer forming the stripe region while using an SiO2 mask formed on the high-resistance regions 46. Further, n-type GaAs regions 47 are formed on the high-resistance regions 46 at both lateral sides of the central stripe region, and a p-type electrode of Cr/Au/Pt/Au structure is formed on the top surface of the p-type GaAs layer 49. Further, an n-type electrode 41 of AuGe/Ni is formed on the bottom surface of the substrate 42.
In such a structure, there is formed a real refractive-index waveguide structure in correspondence to the central ridge stripe. Generally, such a laser diode is fabricated such that the epitaxial layers constituting the laser structure achieves a lattice fitting with the GaAs substrate 42.
On the other hand, the Japanese Laid-Open Patent Publication 5-41560 describes a refractive-index waveguide laser diode that uses a double heterostructure of a mixed crystal of (AlGa)aIn1xe2x88x92aP (0.51 less than axe2x89xa60.73) formed on a GaAs substrate, wherein the foregoing double heterostructure is formed with an intervening lattice misfit relaxation layer having a composition represented as GaPxAs1xe2x88x92x.
FIG. 5 shows the relationship between the band edge energy and the lattice constant for various III-V crystals, wherein the continuous lines represent the band edge energy of the conduction band Ec and the valence band Ev of a GaInP mixed crystal while the broken lines represent the conduction band energy and valence band energy of an AlInP mixed crystal.
Referring to FIG. 5, it can be seen that a mixed crystal of the AlGaInP system can be used for the cladding layer and the active layer as long as the AlGaInP mixed crystal has a composition in which the lattice constant is smaller than that of GaAs. When the composition is chosen as such, the bandgap energy increases and the laser oscillation wavelength shifts in the shorter wavelength direction. Thus, the foregoing Japanese Laid-Open Patent Publication 5-41560 proposes a laser diode that can oscillate at the wavelength shorter than 600 nm, by choosing the composition of the AlGaInP mixed crystal constituting the laser diode.
On the other hand, the relationship of FIG. 5 also indicates the possibility of improvement of performance of the red-wavelength laser diode oscillating in the wavelength range of 600-660nm, by using a mixed crystal of AlGaInP having a lattice constant between those of GaAs and GaP, for the cladding layer and the optical waveguide layer.
Further, laser diodes having a refractive-index waveguide structure with current-blocking regions of GaAs or AlInP are proposed. In such a refractive-index waveguide laser diode, it is also possible to use a mixed crystal of AlGaInP for the current-blocking regions. However, the use of a mixed crystal composition containing a large amount of Al such as AlInP causes a problem to be described later.
In order to fabricate such a real refractive-index waveguide laser diode, it is necessary to form a real refractive-index profile in a transverse direction of the active layer. Normally, this is achieved by forming a ridge-stripe structure or a groove-stripe during the fabrication process of the laser diode by an etching process and by forming a cladding layer or current-blocking regions of AlGaInP, and the like, by a regrowth process.
In the case of forming a layer of AlGaInP on a substrate of GaP, GaAs or a ternary substrate such as GaAsP or GaInP by an MOCVD process, there is a tendency of extensive formation of hillock structure on the surface of the Al GaInP layer thus grown when the AlGaInP layer is grown on the substrate having a (100) principal surface or when the offset angle of the substrate principal surface from the (100) surface is small. This tendency of hillock formation is enhanced when the mixed crystal layer thus grown contains a large amount of Al as in the case of an AlInP mixed crystal.
It is possible to suppress the hillock formation to some extent by using an offset substrate and by increasing the offset angle of the substrate. However, such suppressing of hillock formation by way of using an offset substrate tends to become difficult in the case of an AlGaInP mixed crystal containing a large amount of Al and Ga and hence having a lattice constant smaller than that of GaAs. Further, use of an offset GaAsP substrate having a large offset angle poses a problem of availability as compared with the case of using a readily available industrial standard GaAs substrate.
When such hillock structure exists extensively in the semiconductor layers constituting a laser diode or an LED, the device performance or the yield of device production may be degraded seriously. This problem appears particularly serious in the case of regrowing a mixed crystal containing Al. In such a case, realization of a sufficient crystal quality is extremely difficult due to the surface oxidation of the underlying layer.
In the case of the laser diode disclosed in the Japanese Laid-Open Patent Publication 5-41560, op. cit., it is believed that fabrication of a satisfactory laser diode device with high-quality crystal layers is difficult.
Thus, it is an object of the present invention to provide a laser diode operable in the wavelength range of 600-660 nm wherein the device performance is improved by improving the quality of the crystal constituting the current-blocking regions.
A material of the AlGaInP system is a direct-transition type III-V material having the largest bandgap energy except for a material of the AlGaInN system. The bandgap energy can reach as much as 2.3 eV (540 nm in bandgap wavelength).
Thus, efforts have been made with regard to optical semiconductor devices of the AlGaInN system to provide a high-luminosity, green to red optical source for use in various color display devices or a laser diode for use in laser printers, compact disk drives, DVDs for optical writing of information.
In the case of a laser diode, a material system achieving a lattice matching with a GaAs substrate has conventionally been used. It should be noted that a laser diode for high-density optical recording is required to produce a large optical output of short-wavelength in a high temperature environment.
In order to construct a laser diode, it is necessary to provide a structure for confining both carriers and optical radiation in an active layer or light-emitting layer by using a cladding layer. Thus, a cladding layer is required to have a bandgap larger than a bandgap of the active layer.
In this regard, the material in the system of AlGaInP has a drawback in that the band discontinuity xcex94Ec on the conduction band tends to become smaller. In such a case, the injected carriers easily escape from the active layer into the cladding layer by causing an overflow. When such an overflow of carriers takes place, the threshold current of laser oscillation becomes sensitive with the operational temperature of the laser diode and the temperature characteristic of the laser diode is deteriorated.
In order to overcome the problem, the Japanese Laid-Open Patent Publication 4-114486 proposes a structure that uses an MQB (multiple quantum barrier) structure, in which a large number of extremely thin layers are stacked between the active layer and the cladding layer for carrier confinement. This structure, however, is complex, and it has been difficult to achieve the desired effect in view of the necessity of precision control of thickness of the layers to the degree of atomic layer level.
In an ordinary edge-emission type red-wavelength laser diode that uses a structure in which the active layer is sandwiched by a pair of optical guide layers having a composition represented as (AlxGa1xe2x88x92x)0.5In0.5P, the desired optical confinement is realized in the optical guide layers of the composition (AlxGa1xe2x88x92x)0.5Ino0.5P. On the other hand, the optical guide layers generally contain Al with a composition x of 0.5 or more, while such a high concentration of Al in the optical waveguide layer causes the problem of optical damaging at the optical cavity edge surface of the laser diode due to the recombination of carriers facilitated by Al. Thus, there has been a difficulty in obtaining a high optical output power or realizing a stable operation of the laser diode over a long period of time.
Summarizing above, conventional laser diodes constructed on a GaAs substrate with lattice matching therewith have a problem in operation under high temperature environment, or high-output operation, or operation over a long period of time. For example, it has been difficult to realize a red-wavelength laser diode operable under a high temperature environment such as 80xc2x0 C. with high output power such as 70 mW or more, over a long period of time such as ten thousand hours. The difficulty increases with decreasing output wavelength of the laser diode.
The material of the system of AlGaInP having a lattice constant smaller than the lattice constant of GaAs is characterized by a wide bandgap and is suitable for decreasing the output wavelength of the laser diode or light-emitting diode. Thus, there is a proposal in the Japanese Laid-Open Patent Publication 8-18101 with regard to a light-emitting diode (LED) using the foregoing material system as well as other material systems. Further, there are proposals of a short wavelength laser diode oscillating at a wavelength of 600 nm or less. For example, the Japanese Laid-Open Patent Publication 5-41560 proposes a laser diode in which a double heterostructure having a composition of (AlGa)aIn1xe2x88x92aP (0.51 less than axe2x89xa60.73) and a lattice constant intermediate between GaAs and GaP is provided on a GaAs substrate with an intervening buffer layer of GaPxAs1xe2x88x92x having a composition adjusted so as to achieve a lattice matching with the foregoing double heterostructure. In the foregoing proposal, the problem of lattice misfit is resolved by interposing the buffer layer between the substrate and the double heterostructure.
FIG. 6 shows the relationship between the bandgap energy and the lattice constant for various III-V materials.
Referring to FIG. 6, the continuous lines represent the composition causing a direct-transition, while the broken lines represent the composition causing an indirect-transition. It should be noted that the material of the foregoing composition (AlGa)aIn1xe2x88x92aP (0.51 less than axe2x89xa60.73) having the lattice constant between GaAs and GaP falls in the region defined by the composition of AlInP and the composition of GaInP. By using the material system of AlGaInP having a bandgap larger than the bandgap of the material achieving a lattice matching with a GaAs substrate for the active layer and the cladding layers, it is possible to reduce the oscillation wavelength of the laser diode to be smaller than 600 nm.
FIG. 7 shows the construction of a laser diode having a refractive-index waveguide disclosed in the Japanese Laid-Open Patent Publication 5-41560, wherein the laser diode has a lattice constant between GaAs and GaP.
Referring to FIG. 7, the laser diode is constructed on a substrate 51 of n-type GaAs and includes a graded layer 52 of n-type GaPAs formed on the substrate 51, and a superlattice layer 53 of n-type Ga0.7In0.3P/(Al0.7Ga0.3)0.7In0.3P formed on the graded layer 52, wherein the substrate 51, the graded layer 52 and the superlattice layer 53 form together a GaPAs semiconductor substrate 54. The GaPAs semiconductor substrate 54 thus formed carries thereon consecutively an optical waveguide layer 55 of n-type AlGaInP having a composition of (Al0.7Ga0.3)0.7In0.3P, an active layer 56 of undoped GaInP having a composition of Ga0.7In0.3P, and an optical waveguide layer 57 of p-type AlGaInP having a composition of (Al0.7Ga0.3)0.7In0.3P, and a first buffer layer 58 of p-type GaInP having a composition of Ga0.7In0.3P is provided further on the optical waveguide layer 57.
The first buffer layer 58 and the underlying optical waveguide layer 57 are then subjected to a mesa etching process to form a ridge stripe structure, wherein the mesa etching process is conducted such that the optical waveguide layer 57 is left with a thickness of 0.2-0.4 xcexcm outside the ridge stripe structure.
At both lateral sides of the ridge stripe structure, a pair of current-blocking regions 59 of n-type GaInP having a composition of Ga0.7In0.3P are formed by a regrowth process, wherein the current-blocking regions 59 function also as an optical absorption region. Further, a contact layer 60 of p-type GaInP having a composition of Ga0.7In0.3P is formed on the current-blocking regions 59 including the ridge stripe region formed therebetween, by a regrowth process. Further, p-type electrode 62 and an n-type electrode 61 are formed respectively on the top surface of the contact layer 60 and on the bottom surface of the GaAs substrate 51.
In the foregoing laser diode that uses a material system having a lattice constant between GaP and GaAs, it is necessary to carry out three regrowth process steps, one for growing the GaInP buffer layer 58, one for growing the current-blocking regions 59, and one for growing the contact layer 60. Thereby, the fabrication process of the laser diode is complex and the yield of production tends to be reduced.
In order to facilitate the fabrication of a ridge-waveguide laser diode, there is also a proposal in the Japanese Laid-Open Patent Publication 10-4239, to form the current-blocking regions by way of oxidation of an AlGaAs mixed crystal having a composition represented as AlxGa1xe2x88x92xAs (0.8 less than xxe2x89xa61). According to the foregoing proposal, the ridge structure is formed to have a width of 4 xcexcm at the bottom part thereof, and there is provided a current path region as a non-oxidized part of the AlGaAs region of the foregoing composition of AlxGa1xe2x88x92xAs (0.8 less than xxe2x89xa61), with a width of 3 xcexcm.
According to the foregoing proposal, it is possible to form a laser diode having the current-blocking structure in a single crystal growth process.
On the other hand, the laser diode of the foregoing prior art has a drawback, in view of the difference in the lattice constant between the material system having a lattice constant between GaAs and GaP and the foregoing AlGaAs mixed crystal of the composition AlxGa1xe2x88x92xAs (0.8 less than xxe2x89xa61), which achieves a lattice matching with the GaAs substrate, in that the thickness of the AlGaAs mixed crystal layer of the composition AlxGa1xe2x88x92xAs (0.8 less than xxe2x89xa61) is inevitably limited when the AlGaAs mixed crystal layer is to be provided in the material system having a lattice constant between GaAs and GaP. Further, in view of the fact that the current path region of the not-oxidized AlxGa1xe2x88x92xAs (0.8 less than xxe2x89xa61) mixed crystal layer extends such that the edge of the current path region is located near the edge of the ridge structure, there appears a substantial optical waveguide loss and increase of optical output power is difficult.
Thus, the present invention has an object to provide a semiconductor light-emitting device formed of a semiconductor material having a lattice constant between GaP and GaAs wherein the fabrication process is simplified. Further, the present invention has an object to provide a semiconductor light-emitting device formed of a semiconductor material having a lattice constant between GaP and GaAs wherein the optical waveguide loss is minimized and suitable for increasing output optical power.
Meanwhile, vertical-cavity laser diodes, which emit optical beam in a direction perpendicular to a substrate surface, draw attention in relation to application of red-wavelength optical source in the wavelength range of 630-660 nm for use in high-density optical disk drives and laser printers, in view of the fact that a vertical-cavity laser diode provides various advantageous features such as high-efficiency of laser oscillation, excellent beam property, excellent vertical mode property, and the like. Further, the vertical-cavity laser diodes are suitable for constructing a two dimensional array, and thus, there are possibility of application to the art of optical interconnection or optical array for laser beam printers.
In view of the limited length of optical cavity, a vertical-cavity laser diode requires to provide a large reflectance. Because of this reason, a distributed Bragg reflector (DBR) is generally used as the mirror of the vertical optical cavity. By using a DBR, it is possible to achieve a near 100% reflectance. A DBR is formed by stacking two semiconductor layers or dielectric layers having mutually different refractive index alternately and repeatedly with an optical distance corresponding to a quarter of the oscillation wavelength.
When the difference of refractive index between the two semiconductor layers constituting a DBR is large, a high reflectance is achieved with a reduced number of repetition. In order to avoid optical absorption and to increase the efficiency of laser oscillation, the semiconductor layers constituting the DBR are required to be transparent to the laser oscillation wavelength.
In the case of a vertical-cavity laser diode using the material of an AlGaInP system and oscillating at the wavelength of 630-650 nm, an active layer of GaInP is formed on a GaAs substrate, and a DBR is formed of high refractive layers of AlGaInP and low refractive layers of AlInP.
In view of the tendency of increase of bandgap and decrease of refractive index with increasing Al content in a semiconductor layer containing Al, it is desirable to construct a DBR by stacking AlInP layers and GaInP layers. Unfortunately, a GaInP layer is not transparent to the optical radiation in the wavelength range of 630-650 nm. Thus, there occurs a problem of optical absorption and degradation of optical cavity efficiency.
FIG. 8 shows the relationship between the lattice constant and bandgap for the GaInP and AlInP mixed crystals, wherein FIG. 8 shows the r valley energy and the X valley energy of the conduction band and further the band edge energy of the valence band. As can be seen from FIG. 8, the bandgap energy increases with decreasing lattice constant in the foregoing material system.
In the invention disclosed in the Japanese Laid-Open Patent Publication 9-199793, a DBR is constructed by combining an AlInP/GaInP layered structure formed on a GaAs substrate with a lattice constant smaller than the lattice constant of the substrate and an AlGaAs/GaAs layered structure, for reducing the optical loss caused by the DBR. According to the foregoing prior art, a first DBR structure of the AlGaAs/GaAs layered structure is formed on the GaAs semiconductor substrate and a second DBR structure of the GaInP/AlInP is formed thereon, with a graded layer interposed between the first and second DBR structures for relaxing the lattice misfit. On the DBR thus formed, a first graded cladding layer, a GaInP active layer and a second graded cladding layer are formed such that the composition grading is symmetric between the first and second graded cladding layers. Further, a further DBR structure is formed on the second cladding layer.
The invention disclosed in the foregoing Japanese Laid-Open Patent Publication 9-199793 is designed so as to minimize the optical absorption in the visible wavelength region and to improve the optical cavity efficiency. The two different material systems are used for constructing a DBR to eliminate the problem of lattice misfit of the AlGaInP mixed crystal and for avoiding the difficulty of growing a high quality AlGaInP mixed crystal layer. The difficulty of growing an AlGaInP layer will be explained later. Thus, the foregoing prior art uses the material system of AlGaInP for the DBR structure in the vicinity of the active layer where the intensity of optical radiation is large and uses the material system of AlGaAs for the DBR structure in the part away from the active layer in order to avoid the problem of degradation of the crystal quality associated with the increase of the number of stacks.
Further, there is another prior art vertical-cavity laser diode disclosed in the Japanese Laid-Open Patent Publication 10-200202 wherein the vertical-cavity laser diode of this prior art is constructed on a GaInP substrate.
According to this prior art, a substrate of GaInP having a composition of Ga0.75In0.25P is used and a DBR of the AlInP/GaInP is formed thereon with lattice matching. On the DBR thus formed, an active layer of GaInP is formed. According to this prior art, the problem of degradation of the crystal quality associated with lattice misfit is improved.
In the case of the forgoing prior art device of the Japanese Laid-Open Patent Publication 9-199793, it should be noted a plurality of DBR structures having different lattice constants are provided in a single laser diode device for changing the lattice constant. Further, in view of the fact that the DBR structure that causes a lattice misfit with the substrate has a large thickness, the use of the lattice misfit relaxation layer is not effective for improving the crystal quality. It should be noted that the DBR structure that causes a lattice misfit with the substrate contains at least 20 pairs of layers (40 layers or more) therein.
In the case of the laser diode disclosed in the Japanese Laid-Open Patent Publication 200202, a lattice matching is successfully achieved with respect to the GaInP layer transparent to the optical radiation in the wavelength range of 635-660 nm by choosing the lattice constant of the substrate to be smaller than the lattice constant of GaAs. On the other hand, the laser diode of the foregoing prior art has a drawback in that increase of Al or Ga content in the AlInP or GaInP material system facilitates hillock formation. Particularly, increase of Al content causes an extensive hillock formation and causes a serious problem in the AlInP material. There is no fundamental solution to this problem of hillock formation. When such defects are formed, the homogeneity of the heteroepitaxial interface is degraded substantially, and the optical scattering associated with such a poor quality interface increases the optical loss. Thereby, the optical cavity efficiency is deteriorated.
Further, the invention disclosed in the foregoing Japanese Laid-Open Patent Publication 10-200202 has a drawback, associated with the use of the GaInP active layer, in that there is a limitation imposed over the lattice constant when the laser diode is to be operated in the wavelength range of 630-650 nm.
More specifically, the wavelength of the GaInP mixed crystal that achieves lattice matching with GaAs is about 650 nm, and the wavelength becomes shorter when a GaInP mixed crystal having a lattice constant smaller than that of GaAs is used for the active layer. In order to achieve the foregoing desired wavelength range, it is therefore necessary to reduce the Ga content so as to increase the oscillation wavelength of the laser diode. However, such a decrease of the Ga content causes a compressive strain in the active layer and the quality of the crystal of the active layer is deteriorated. Thus, the lattice constant of the active layer is practically limited to the range close to the lattice constant of GaAs and the degree of freedom in designing the laser oscillation wavelength is limited.
On the other hand, the foregoing construction of the Japanese Laid-Open Patent Publication provides a possibility of increasing the degree of freedom in the laser diode design associated with the deviation of lattice constant from the lattice constant of GaAs, such as increased degree of freedom in selecting the material for various parts of the laser diode. It should be noted that the laser diode of the foregoing Japanese Laid-Open Patent Publication 10-200202 merely focuses on the problem of the optical absorption of the DBR, and no further proposals are made with regard to the improvement of other aspects of the laser diode.
There are further rooms for improvement in the vertical-cavity laser diode having a lattice constant between GaAs and GaP.
Thus, the present invention provides a vertical-cavity laser diode operable in the wavelength range of 630-660 nm and various optical systems using such a vertical-cavity laser diode.
Accordingly, it is a general object of the present invention to provide a novel and useful laser diode, a vertical-cavity laser diode and an optical semiconductor device wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a red-wavelength laser diode having a reduced optical waveguide loss and simultaneously a reduced optical damage at an edge surface of an optical cavity formed in the laser diode.
Another object of the present invention is to provide a laser diode, comprising:
a substrate of a first conductivity type, said substrate having a lattice constant of GaAs or a lattice constant between GaAs and GaP;
a first cladding layer of AlGaInP having said first conductivity type formed over said substrate;
an active layer of GaInAsP formed over said first cladding layer;
an etching stopper layer of GaInP formed over said active layer;
a pair of current-blocking regions of AlGaInP formed over said etching stopper layer, said pair of current-blocking regions defining therebetween a strip region;
an optical waveguide layer of AlGaInP formed over said pair of current-blocking regions so as to include said stripe regions, said optical waveguide layer covering said etching stopper layer in said stripe region; and
a second cladding layer of AlGaInP of a second conductivity type formed over said optical waveguide layer;
said current-blocking regions having an Al content substantially identical with an Al content of said second cladding layer.
According to the present invention, the real refractive-index increases in correspondence to the strip region where the optical waveguide layer of AlGaInP is formed, and the laser diode has a real refractive-index waveguide structure characterized by a low optical loss. Due to the fact that the current-blocking regions outside the stripe region are formed of AlGaInP characterized by a large bandgap, the optical loss caused by such current-blocking regions is successfully minimized. In view of the fact that the AlGaInP current-blocking regions contain Al with a concentration substantially identical with the second cladding layer, which is also formed of AlGaInP, there is no increase of Al content in these parts of the laser diode. Thereby, the problem of damaging at the edge surface of the laser optical cavity caused by non-optical recombination of carriers, is successfully minimized.
Another object of the present invention is to provide a laser diode, comprising:
a substrate having a lattice constant between GaAs and GaP, said substrate having a first conductivity type;
a first cladding layer of AlGaInP having said first conductivity type formed over said substrate;
a lower optical waveguide layer of GaInP formed over said first cladding layer;
an active layer of GaInAsP formed over said lower optical waveguide layer;
a first upper optical waveguide layer of GaInP formed over said active layer;
a pair of current-blocking regions of AlGaInP formed over said first upper optical waveguide layer, said pair of current-blocking regions defining therebetween a stripe region;
a second upper optical waveguide layer of AlGaInP formed over said pair of current-blocking regions so as to include said stripe regions, said second upper optical waveguide layer covering said first upper optical waveguide layer in said stripe region; and
a second cladding layer of AlGaInP having a second conductivity type formed over said second upper optical waveguide layer;
said current-blocking regions having an Al content generally identical with an Al content of said second cladding layer.
According to the present invention, the laser diode has an SCH structure in which the active layer is sandwiched vertically by the lower optical waveguide layer and the first upper optical waveguide layer both free from Al. Thereby, the problem of optical damaging at the edge surface of the laser optical cavity is successfully avoided.
Another object of the present invention is to provide a laser diode, comprising:
a substrate having a lattice constant between GaAs and GaP, said substrate having a first conductivity type;
a first cladding layer of AlGaInP having said first conductivity type formed over said substrate;
a lower optical waveguide layer of GaInP formed over said first cladding layer;
an active layer of GaInAsP formed over said lower optical waveguide layer;
a first upper optical waveguide layer formed over said active layer;
a pair of current-blocking regions of AlGaInP formed over said first upper optical waveguide layer, said pair of current-blocking regions defining therebetween a stripe region;
a second upper optical waveguide layer of GaInP formed over said pair of current-blocking regions so as to include said stripe regions, said second upper optical waveguide layer covering said first upper optical waveguide layer in said stripe region; and
a second cladding layer of AlGaInP having a second conductivity type formed over said second upper optical waveguide layer;
said current-blocking regions having an Al content generally identical with an Al content of said second cladding layer,
said first upper optical waveguide layer of GaInP and said second upper optical waveguide layer of GaInP having respective thicknesses such that a sum of said thickness of said first upper optical waveguide layer and said thickness of said second upper optical waveguide layer is equal to a thickness of said lower optical waveguide layer of GaInP.
According to the present invention, the vertical distribution profile of refractive-index becomes substantially symmetric about the active layer due the fact that the first and second upper optical waveguide layers of GaInP have the total thickness generally identical with the thickness of the lower optical waveguide layer of GaInP. Thereby, the optical radiation produced by the laser diode is effectively confined at the central part of the laser structure and the threshold of laser oscillation can be reduced.
Another object of the present invention is to provide a laser diode operable in the wavelength range of 600-660 nm wherein the device performance is improved by improving the quality of the crystal constituting the current-blocking regions.
Another object of the present invention is to provide a laser diode, comprising:
a substrate having a first conductivity type;
a first cladding layer of said first conductivity type provided over said substrate, said first cladding layer having a lattice constant between GaAs and GaP;
an active layer formed over said first cladding layer;
a second cladding layer of a second conductivity type provided over said active layer, said second cladding layer having said lattice constant;
a ridge-stripe region formed in said second cladding layer; and
a pair of current-blocking regions of said first conductivity type respectively provided over said second cladding layer at both lateral sides of said ridge-stripe region;
each of said current-blocking regions having a composition represented as (Alx1Ga1xe2x88x92x1)y1In1xe2x88x92y1Asz1P1xe2x88x92z1 (0xe2x89xa6x1xe2x89xa61, 0xe2x89xa6y1xe2x89xa61, 0.01xe2x89xa6z1xe2x89xa61).
Another object of the present invention is to provide a laser diode, comprising:
a substrate having a first conductivity type;
a first cladding layer of said first conductivity type provided over said substrate, said first cladding layer having a lattice constant between GaAs and GaP;
an active layer formed over said first cladding layer;
a second cladding layer of a second conductivity type provided over said active layer, said second cladding layer having said lattice constant;
a current-blocking layer of said first conductivity type respectively provided over said second cladding layer;
a stripe depression formed in said current-blocking layer; and
a third cladding layer of said second conductivity type formed over said current-blocking layer so as to include said stripe depression,
said current-blocking layer having a composition represented as (Alx2Ga1xe2x88x92x2)y2In1xe2x88x92y2Asz2P1xe2x88x92z2 (0xe2x89xa6x2xe2x89xa61, 0xe2x89xa6y2xe2x89xa61, 0.01xe2x89xa6z2xe2x89xa61).
According to the present invention, the hillock formation is successfully suppressed by incorporating As into said current-blocking regions or current-blocking layer.
It should be noted that the inventor of the present invention has discovered that the hillock formation is successfully suppressed by incorporating As when growing an AlGaInP mixed crystal layer. It is also discovered that the hillock formation can be reduced by suitably choosing the condition of growth of the AlGaInP mixed crystal layer such as increasing growth temperature from 700xc2x0 C. to 750xc2x0 C. However, the optimization of the growth condition was not sufficient for decreasing the hillock formation to the desired level of hillock density. By adding As, on the other hand, a remarkable decrease was observed for the hillock density, even in such a case the growth is conducted at the temperature of 700xc2x0 C. It is believed that As atoms thus incorporated successfully suppressed the droplet formation of Al or Ga during the process of growing the AlGaInP layer.
It was further observed that the suppression of hillock formation by As is effective even in such a case in which the amount of the As atoms incorporated is very small. Naturally, the effect of suppressing hillock formation increases with increasing amount of As in the AlGaInP layer.
It should be noted that the foregoing suppression of hillock formation during the growth process of an AlGaInP mixed crystal layer by way of incorporating As is particularly effective when a substrate having a small offset angle, such as a commercially available GaAsP substrate, is used.
Thus, the laser diode of the present invention has an improved reliability and lifetime as a result of use of an AlGaInAsP mixed crystal containing As for the current-blocking regions or for the current-blocking layer. By using the AlGaInAsP mixed crystal for the current-blocking regions or the current-blocking layer, the flatness and crystal quality of the device surface are improved. Further, the use of the AlGaInAsP mixed crystal is effective for reducing the leakage current path which is formed inside the laser diode as a result of the hillock formation. Further, the decrease of the hillock density reduces the optical scattering in the current-blocking regions and the waveguide loss of the laser diode is reduced accordingly. Thereby, the threshold current of laser oscillation is reduced.
By using a material transparent to the laser beam produced by the laser diode, in other words by using a material having a bandgap larger than a bandgap of the active layer, for the current-blocking regions or layer, the optical absorption outside the current path region of the laser diode is reduced. Thereby, the threshold current of laser oscillation is reduced and the efficiency of laser oscillation is improved. Further, in view of the fact that the AlGaInAsP current-blocking regions or layer, containing a large amount of Al, forms a real refractive-index waveguide structure with the second cladding layer. It should be noted that the current-blocking regions have a smaller refractive-index as compared with the second cladding layer. Thereby, the optical radiation is effectively confined in the stripe region of the laser diode, and the lateral mode of laser oscillation is stabilized. Associated with this, the astigmatism of the laser diode is reduced.
Further, by providing a GaInAsP layer in the second cladding layer or on the current-blocking layer of AlGaInAsP, the GaInAsP layer functions as an etching stopper layer with respect to the etching process applied to the second cladding layer or the current-blocking layer, and the process of forming the stripe ridge structure in the second cladding layer or the process of forming the stripe groove structure in the current-blocking layer as a result of a wet etching process, is facilitated substantially. As a result of use of the GaInAsP etching stopper layer, the height of the stripe ridge structure or the depth of the stripe groove structure is controlled exactly. Further, the use of the GaInAsP etching stopper layer protects the surface of the second cladding layer or the current-blocking layer from being exposed to the air after the etching process, and the problem of surface oxidation of Al in the second cladding layer or in the current-blocking layer is successfully avoided. It should be noted that a GaAsP composition acts as an effective etching stopper against an etching process applied to an AlGaInAsP layer by a hydrochloric acid etchant, while a GaInP composition acts as an effective etching stopper against an etching process applied to an AlGaInAsP layer by a phosphoric or sulfuric acid etchant.
Further, the use of the optical waveguide layer of GaInP adjacent to the active layer eliminates the direct contact of the active layer and the cladding layer that contains Al, and the problem of damaging of the laser cavity edge surface caused by Al is effectively eliminated. Thereby, it becomes possible to operate the laser diode with a high output power.
Another object of the present invention is to provide a semiconductor light-emitting device formed of a semiconductor material having a lattice constant between GaP and GaAs wherein the fabrication process is simplified.
Another object of the present invention is to provide a semiconductor light-emitting device formed of a semiconductor material having a lattice constant between GaP and GaAs wherein the optical waveguide loss is minimized and suitable for increasing output optical power.
Another object of the present invention is to provide a semiconductor light-emitting device, comprising:
a semiconductor substrate;
an active layer provided over said semiconductor substrate, said active layer emitting optical radiation;
a semiconductor layer vertically sandwiching said active layer with another semiconductor layer, said semiconductor layer having a bandgap larger than a bandgap of said active layer and a lattice constant between GaP and GaAs,
said semiconductor layer containing a to-be-oxidized layer in a part thereof with a composition represented as AlxGayIn1xe2x88x92xxe2x88x92yPtAs1xe2x88x92t(0.8xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa60.2, 0xe2x89xa6txe2x89xa61),
a part of said to be-oxidized layer being oxidized to form a selective oxidation region.
According to the present invention, the selective oxidation region forms a current-blocking structure for confining an injected electric current. In view of the fact that the selective oxidation region thus formed has a reduced refractive index, there appears a difference in the real refractive index between the part of the AlGaInPAs to-be-oxidized layer where the oxidized region is formed and the current path region where no such oxidized region is formed. In other words, the current-blocking structure thus formed by the selective oxidation of the AlGaInPAs layer functions also as the real refractive index waveguide structure effective for lateral mode control. As the real refractive index waveguide structure thus formed contains, in the vicinity of the active layer, only the material which is free from waveguide less for all the wavelengths, the laser diode is easily operated to produce a large output optical power.
It should be noted that the foregoing advantageous structure can be formed by a single crystal growth process. Thereby, the semiconductor light-emitting device of the present invention can be fabricated easily with high yield.
Another object of the present invention is to provide a semiconductor light-emitting device, comprising:
a semiconductor substrate;
an active layer provided over said semiconductor substrate, said active layer producing optical radiation; and
a pair of cladding layers sandwiching said active layer vertically,
said active layer being one of a single quantum well structure containing therein a quantum well layer and a multiple quantum well structure containing therein a quantum well layer and a barrier layer,
said quantum well layer comprising a mixed crystal of AlGaInPAs having a composition represented as (Alx1Ga1xe2x88x92x1)xcex11In1xe2x88x92xcex11Pt1As1xe2x88x92t1 (0xe2x89xa6x1 less than 1, 0  less than xcex11xe2x89xa61, 0xe2x89xa6t1xe2x89xa61),
said barrier layer comprising a mixed crystal of AlGaInPAs having a composition represented as (Alx2Ga1xe2x88x92x2)xcex12In1xe2x88x92xcex12Pt2As1xe2x88x92t2 (0xe2x89xa6x2 less than 1, 0.5 less than xcex12 less than 1, 0xe2x89xa6t2xe2x89xa61),
each of said cladding layers comprising a mixed crystal of AlGaInPAs containing Al and having a composition represented as (AlyGa1xe2x88x92y)xcex2In1xe2x88x92xcex2PvAs1xe2x88x92v (0 less than yxe2x89xa61, 0.5 less than xcex2 less than 1, 0 less than vxe2x89xa61), each of said cladding layers having a lattice constant between GaP and GaAs and a bandgap larger than a bandgap of said active layer,
an optical waveguide layer of AlGaInPAs interposed between said active layer and each of said cladding layers, said optical waveguide layer having a bandgap larger than the bandgap of said active layer but smaller than the bandgap of said cladding layer, said optical waveguide layer having a composition represented as (AlzGa1xe2x88x92z)65 In1xe2x88x92xcex3PuAs1xe2x88x92u(0xe2x89xa6z less than 1, 0.5 less than xcex3 less than 1, 0 less than uxe2x89xa61),
a to-be-oxidized layer provided in at least one of said cladding layers such that said cladding layer contains said to-be-oxidized layer in correspondence to a part thereof, or between said active layer and one of said cladding layers, said to-be-oxidized layer having a composition represented as AlxGayIn1xe2x88x92xxe2x88x92yPtAs1xe2x88x92t (0.8xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa60.2, 0xe2x89xa6txe2x89xa61), a part of said to-be-oxidized layer being selectively oxidized to form a selective oxidized region.
According to the present invention, it is possible to oscillate the laser diode in the visible wavelength band in view of the fact that the active layer is formed of a mixed crystal of AlGaInPAs having a composition represented as (Alx1Ga1xe2x88x92x1)xcex11In1xe2x88x92xcex11Pt1As1xe2x88x92t1 (0xe2x89xa6x1 less than 1, 0 less than xcex11xe2x89xa61, 0xe2x89xa6t1xe2x89xa61). In view of the fact that a mixed crystal of AlGaInPAs containing Al with a composition represented as (AlyGa1xe2x88x92y)xcex2In1xe2x88x92xcex2PvAs1xe2x88x92v (0 less than yxe2x89xa61, 0.5 less than xcex2 less than 1, 0 less than vxe2x89xa61) and having a lattice constant between GaP and GaAs is used for the cladding layer, the bandgap of the cladding layer is increased as compared with the case of using a cladding layer having a lattice matching composition to the GaAs substrate, and the wavelength of the output optical radiation of the semiconductor light-emitting device is reduced.
Further, in view of the fact that the semiconductor light-emitting device of the present invention employs the an SCH structure in which the mixed crystal of (Alx1Ga1xe2x88x92x1)xcex11In1xe2x88x92xcex11Pt1As1xe2x88x92t1 (0xe2x89xa6x1 less than 1, 0 less than xcex11xe2x89xa61, 0xe2x89xa6t1xe2x89xa61) is used for the quantum well layer forming the active layer and in which the mixed crystal of (AlzGa1xe2x88x92z)xcex3In1xe2x88x92xcex3PuAs1xe2x88x92u (0xe2x89xa6z less than 1, 0.5 less than xcex3 less than 1, 0 less than uxe2x89xa61) is used for the optical waveguide layer, a wide bandgap can be realized with a reduced Al content as compared with the case of using a material forming a lattice matching with a GaAs substrate, and the non-optical recombination of carriers is reduced substantially. Associated with this, the efficiency of optical emission is improved. In the case of a laser diode, the problem of damaging of optical cavity edge surface as a result of the non-optical recombination of carries is reduced and the laser diode can be operated stably and reliably with high optical output power.
Further, it is possible to induce a strain in the semiconductor light-emitting device of the present invention with respect to the cladding layer. In this case, bandgap of the active layer can be reduced. Further, a large conduction band discontinuity can be realized in the semiconductor light-emitting device of the present invention by reducing the Al content in the optical waveguide layer. Thereby, the problem, pertinent to a conventional red-wavelength laser diode of the AlGaInP system, of carrier overflow taking place on the conduction band, is reduced substantially.
By interposing the to-be-oxidized layer in a part of one or both of the two cladding layers or at the interface between the active layer and one of the cladding layers with the composition of AlxGayIn1xe2x88x92xxe2x88x92yPtAs1xe2x88x92t (0.8xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa60.2, 0xe2x89xa6txe2x89xa61), followed by an oxidizing process oxidizing a part of the to-be-oxidized layer, it is possible to form a current-blocking region by the oxidized region thus formed selectively in the to-be-oxidized layer. As the oxidized region thus formed has a reduced refractive index, there is also formed a real refractive index waveguide structure by the part of the to-be-oxidized region where the selective oxidation has occurred and by the part where no such a selective oxidation has occurred. Thereby the lateral mode control becomes possible in the semiconductor light-emitting device. The real refractive index waveguide structure thus formed contains, in the vicinity of the active layer, only the material which is free from waveguide less for all the wavelengths, the laser diode is easily operated to produce a large output optical power.
It should be noted that the foregoing advantageous structure can be formed by a single crystal growth process. Thereby, the semiconductor light-emitting device of the present invention can be fabricated easily with high yield.
Another object of the present invention is to provide a semiconductor light-emitting device, comprising:
a GaAs substrate;
an active layer provided over said GaAs substrate, said active layer emitting an optical radiation;
a pair of semiconductor layers sandwiching said active layer vertically,
said semiconductor layer containing a to-be-oxidized layer in a part thereof with a composition represented as AlxGayIn1xe2x88x92xxe2x88x92yPtAs1xe2x88x92t(0.8xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa60.2, 0xe2x89xa6txe2x89xa61) and containing P,
a part of said to be-oxidized layer being oxidized to form a selective oxidation region.
According to the present invention, it is possible to achieve a lattice matching for the to-be-oxidized layer with respect to the GaAs substrate by introducing P therein. Thereby, the adversary effect of strain caused in the to-be-oxidized layer is effectively eliminated.
Another object of the present invention is to provide a semiconductor light-emitting device, comprising:
a GaAs substrate;
an active layer of an AlGaInP system formed over said GaAs substrate, said active layer emitting optical radiation;
a pair of semiconductor layers sandwiching said active layer vertically, each of said semiconductor layers having a bandgap larger than a bandgap of said active layer,
said semiconductor layers including, in a part thereof, a layer of AlGaInAs having a composition represented as AlxGayIn1xe2x88x92xxe2x88x92yAs (0.8xe2x89xa6xxe2x89xa61,0xe2x89xa6yxe2x89xa60.2),
a part of said semiconductor layer being oxidized to form a pair of oxidized regions, with a not-oxidized region formed therebetween with a width w1, a total width of said pair of oxidized regions being defined as w2,
wherein said width w1 is set such that a ratio of said width w1 with respect to a sum of said width w1 and said width w2, defined as w1/(w1+w2) is smaller than 0.6.
According to the present invention, the waveguide loss caused by the variation of the edge width is successfully eliminated by setting the foregoing width to be smaller than 0.6.
Another object of the present invention is to provide a semiconductor light-emitting device, comprising:
a GaAs substrate;
an active layer provided over said GaAs substrate, said active layer emitting optical radiation; and
a pair of semiconductor layers sandwiching said active layer vertically, each of said semiconductor layers having a bandgap larger than a bandgap of said active layer,
said semiconductor layers including, in a part thereof, a layer of AlGaInAs having a composition represented as AlxGayIn1xe2x88x92xxe2x88x92yAs (0.8xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa60.2),
a part of said semiconductor layer being oxidized to form an oxidized region,
a ridge structure being formed in a part of said semiconductor layer located at least above said layer of AlGaInAs, said ridge structure having a ridge width exceeding 10 xcexcm.
According to the present invention, a large contact area for electrode is secured by setting the ridge width to be larger than 10 xcexcm and the differential resistance during the device operation is reduced. Further, the structure is suitable for a flip-chip mounting in which the heat of the active region is efficiently dissipated to a supporting substrate via the electrode.
Another object of the present invention is to provide a vertical-cavity laser diode operable in the wavelength range of 630-660 nm and various optical systems using such a vertical-cavity laser diode.
Another object of the present invention is to provide a vertical-cavity laser diode, comprising:
a substrate;
an active layer provided over said substrate, said active layer emitting optical radiation; and
a distributed Bragg reflector provided over said substrate in an optical path of said optical radiation emitted from said active layer in a direction perpendicularly to a plane of said active layer, said distributed Bragg reflector comprising a plurality of layers stacked over said substrate,
said distributed Bragg reflector having a lattice constant between GaAs and GaP and including at least two semiconductor layers of respective, mutually different compositions,
at least one of said semiconductor layers having a composition represented as (Alx1Ga1xe2x88x92x1)y1In1xe2x88x92y1Asz1P1xe2x88x92z1 (0xe2x89xa6x1xe2x89xa61, 0.5xe2x89xa6y1xe2x89xa61, 0 less than z1 less than 1).
According to the present invention, the distributed Bragg reflector (DBR) is formed of an AlInAsP mixed crystal containing therein As. Thereby, the hillock formation on the surface of the layers constituting the DBR is substantially completely suppressed. Thereby, the problem of optical loss associated with the hillocks formed in the DBR is eliminated and the reflectance of the DBR is improved remarkably. With the improvement in the reflectance of the DBR, the oscillation threshold of the laser diode is improved and the device performance and device lifetime are improved also. The improvement becomes appreciable when As is added with a concentration of about 1%.
Another object of the present invention is to provide a vertical-cavity laser diode, comprising:
a substrate;
an active layer provided over said substrate, said active layer emitting optical radiation; and
a distributed Bragg reflector provided over said substrate in an optical path of said optical radiation emitted from said active layer in a direction perpendicularly to a plane of said active layer, said distributed Bragg reflector comprising a plurality of layers stacked over said substrate,
said active layer having a composition represented as Ga1xe2x88x92x2In1xe2x88x92y2Asz2P1xe2x88x92z2 (0xe2x89xa6y2xe2x89xa61, 0xe2x89xa6z2xe2x89xa61).
According to the present invention, the desired laser oscillation wavelength is realized with reduced strain as a result of use of GaInAsP for the active layer. Thereby, the surface morphology of the active layer is improved, and the efficiency of laser oscillation is improved as a result of the improvement of quality of crystal of the active layer. Further, as a result of reduced strain in the active layer, the degree of freedom for designing the laser diode is improved. As the active layer has a lattice constant closer to GaP or AlP as compared with the prior art vertical-cavity laser diode, it becomes possible to use a layer of AlInAsP for the DBR. As the layer of AlInAsP has a small refractive index, the number of stacks of the layers in the DBR is reduced, and the resistance of the laser diode is accordingly reduced.
Another object of the present invention is to provide a vertical-cavity laser diode, comprising:
a substrate;
an active layer provided over said substrate, said active layer emitting optical radiation;
a distributed Bragg reflector provided over said substrate in an optical path of said optical radiation emitted from said active layer in a direction perpendicularly to a plane of said active layer, said distributed Bragg reflector having a lattice constant between GaAs and GaP; and
a pair of semiconductor layers having a composition represented as Gay3In1xe2x88x92y3P (0.5xe2x89xa6y3xe2x89xa61) provided at upper and lower sides of said active layer.
According to the present invention, it is possible to reduce the number of non-optical recombination centers associated with Al by providing the GaInP layers at both upper and lower sides of the active layer. Further, the problem of multiplication of crystal defects originating from Al, or the problem of migration of the crystal defects into the active region of the laser diode, is also reduced and the reliability of the laser diode is improved.
Another object of the present invention is to provide a vertical-cavity laser diode, comprising:
a substrate;
an active layer provided over said substrate, said active layer emitting optical radiation;
a distributed Bragg reflector provided over said substrate in an optical path of said optical radiation emitted from said active layer in a direction perpendicularly to a plane of said active layer, said distributed Bragg reflector having a lattice constant between GaAs and GaP;
a contact layer provided over said distributed Bragg reflector; and
an electrode provided on said contact layer in ohmic contact therewith,
said contact layer being transparent to an optical beam produced as a result of interaction of said optical radiation produced by said active layer with said distributed Bragg reflector.
According to the present invention, the process of eliminating a part of the contact layer in correspondence to an optical window, from which the optical beam is emitted to the region outside the laser diode, is eliminated as a result of use of a material transparent to the optical beam for the contact layer.
Another object of the present invention is to provide a vertical-cavity laser diode, comprising:
a substrate;
an active layer provided over said substrate, said active layer emitting optical radiation; and
a distributed Bragg reflector provided over said substrate in an optical path of said optical radiation emitted from said active layer in a direction perpendicularly to a plane of said active layer, said distributed Bragg reflector having a lattice constant between GaAs and GaP,
said distributed Bragg reflector including therein a semiconductor layer having a composition represented as AlAsz4P1xe2x88x92z4 (0xe2x89xa6z4xe2x89xa61).
According to the present invention that uses AlAsP characterized by a small refractive index as compared with AlInP of the same lattice constant, in becomes possible to increase the refraction index different or step inside the DBR and the number of stacks of layers in the DBR can be reduced. Associated therewith, the threshold current of laser oscillation is reduced together with the device resistance.
Another object of the present invention is to provide a vertical-cavity laser diode, comprising:
a substrate;
an active layer provided over said substrate, said active layer emitting optical radiation; and
a distributed Bragg reflector provided over said substrate in an optical path of said optical radiation emitted from said active layer in a direction perpendicularly to a plane of said active layer, said distributed Bragg reflector having a lattice constant between GaAs and GaP,
said distributed Bragg reflector including therein a semiconductor layer having a composition represented as AlAsz5P1xe2x88x92z5 (0xe2x89xa6z5xe2x89xa61),
said semiconductor layer being laterally sandwiched by a pair of oxide regions formed coplanar to said semiconductor layer, said semiconductor layer and said pair of oxide regions forming a current confinement structure.
According to the present invention, the oxidized regions are formed in the form of high-quality insulator by selective oxidation process of a semiconductor layer containing Al. Larger the Al content, easier for the selective oxidation process. Particularly, an AlAsP mixed crystal, which contains Al as the sole group III element, is easy for oxidation. According to the present invention, the threshold current of laser oscillation is decreased as a result of formation of the current confinement structure.
Another object of the present invention is to provide a vertical-cavity laser diode, comprising:
a substrate;
an active layer provided over said substrate, said active layer emitting optical radiation;
a distributed Bragg reflector provided over said substrate in an optical path of said optical radiation emitted perpendicularly to a plane of said active layer, said distributed Bragg reflector having a lattice constant between GaAs and GaP; and
a semiconductor layer interposed between said active layer and said distributed Bragg reflector, said semiconductor layer having a composition represented as AlAsz6P1xe2x88x92z6 (0xe2x89xa6z6xe2x89xa61),
said semiconductor layer being laterally sandwiched by a pair of oxidized regions formed coplanar to said semiconductor layer.
According to the present invention, a current confinement structure is formed between the DBR and the active layer by applying a selective oxidation process to the semiconductor layer. As the current confinement structure is thus formed in the vicinity of the active layer, the current is injected to the active layer in the form of highly confined state, and lateral spreading of the carriers in the active layer is effectively suppressed. Further, the refractive index distribution in the layer containing the semiconductor layer and the oxidized regions enables an effective control of lateral mode of laser oscillation. Thus, the laser diode of the present invention oscillates at low threshold current with a stabilized lateral mode.
Other objects and further features of the present invention will become apparent from the following detailed description when read in conjunction with the attached drawings.